1. Field of the Invention
The present invention relates to an FM/AM stereo receiver which is capable of receiving both FM and AM stereo broadcasting signals.
2. Description of the Prior Art
An FM/AM stereo receiver has an FM stereo receiving system for receiving an FM stereo broadcasting signal and an AM stereo receiving system for receiving an AM stereo broadcasting signal.
The FM stereo receiving system receives an RF FM stereo signal of the standard FM stereo broadcasting system now in use. In the RF FM stereo signal, a main carrier of a radio frequency (a frequency in an FM broadcasting frequency band ranging from 88.0 to 108.0 MHz in U.S.A. and 76.0 to 90.0 MHz in Japan) is frequency modulated by a composite stereo signal. The composite signal includes a main channel signal, a subchannel signal lying in a frequency band (the band width of which is 15 KHz) higher than that of the main channel signal and a pilot tone lying between the frequency band of the main channel signal and the frequency band (the band width of which is 30 KHz) of the subchannel signal. The main channel signal is composed of a sum signal (L+R) of a left signal (L) having an audio frequency and a right signal (R). In the subchannel signal, a subcarrier of a frequency (38 KHz) sufficiently lower than that of the main carrier is amplitude modulated by a difference signal (L-R) of the left signal (L) and the right signal (R) in an AM suppressed carrier system. The pilot tone has a frequency (19 KHz) which is one-half of the frequency of the subcarrier.
The AM stereo receiving system receives an RF AM stereo signal. The AM stereo broadcasting is now under study for operation. One of RF AF stereo signals of promising AM stereo broadcasting systems is an RF AM stereo signal of an AM-PM system (such as Magnavox system) and another one is an RF AM stereo signal of an AM-FM system (such as Belar system).
In the RF AM stereo signal of the AM-PM system, a phase-modulated signal, obtained by phase modulating a carrier of the radio frequency (a frequency in an AM broadcasting frequency band, that is, in the range of 535 to 1605 KHz in U.S.A. and Japan) by the difference signal (L-R), is amplitude modulated by the sum signal (L+R). In the RF AM stereo signal of the AM-FM system, a frequency-modulated carrier, obtained by frequency modulating the carrier of the radio frequency by the difference signal (L-R), is amplitude modulated by the sum signal (L+R).
In an FM/AM stereo receiver that has heretofore been proposed for receiving the RF FM stereo signal and the RF AM stereo signal based on the AM-PM (or AM-FM) system, an FM stereo receiving system comprises an RF FM stereo tuner for selecting one of RF FM stereo signals and converting the selected RF FM stereo signal into an IF FM stereo signal the non-frequency-modulated main carrier of which has an intermediate frequency (such as 10.8 MHz) lower than the radio frequency of the non-frequency-modulated main carrier of the RF FM stereo signal, an IF FM stereo signal amplifier for amplifying the IF FM stereo signal from the tuner, a limiter for amplitude limiting the IF FM stereo signal from the amplifier, an FM demodulator for frequency demodulating the IF FM stereo signal from the limiter into a composite signal, and a stereo signal demodulator for deriving a right signal (R) and a left signal (L) from the composite signal.
An AM stereo receiving system of the prior art FM/AM stereo receiver comprises an RF AM stereo tuner for selecting one of RF AM stereo signals and converting the selected RF AM stereo signal into an IF AM stereo signal the non-phase- (or frequency-) modulated carrier of which has an intermediate frequency (such as 450 KHz) lower than the radio frequency of the non-phase- (or frequency-) modulated carrier of the RF AM stereo signal, an IF AM stereo signal amplifier for amplifying the IF AM stereo signal from the tuner, an AM demodulator for amplitude demodulating the IF AM stereo signal from the amplifier into a sum signal (R+L), a limiter for amplitude limiting the IF AM stereo signal to obtain such an IF AM (or FM) difference signal that its carrier of a frequency equal to that of the non-phase- (or frequency-) modulated carrier has been phase (or frequency) modulated by a difference signal (L-R), a PM (or FM) demodulator for phase (or frequency) demodulating the IF PM (or FM) difference signal from the limiter into the difference signal (L-R), and a matrix circuit for producing a right signal (R) and a left signal (L) by the sum signal (L+R) from the AM demodulator and the difference signal (L-R) from the PM (or FM) demodulator.
Accordingly, the conventional FM/AM stereo receiver for receiving the RF FM stereo signal and the RF AM stereo signal based on the AM-PM (or AM-FM) system includes two limiters, one for limiting the IF FM stereo signal in the FM stereo receiving system and the other for obtaining the IF PM (or FM) difference signal in the AM stereo receiving system. This is an obstacle to the manufacture of a simple, small and inexpensive FM/AM stereo receiver. Furthermore, the prior art FM/AM stereo receiver necessitates two demodulators, i.e. the FM demodulator for obtaining the composite signal in the FM stereo receiving system and the PM (or FM) demodulator for obtaining the difference signal in the AM stereo receiving system. This is another obstacle to the fabrication of the simple, small and inexpensive FM/AM stereo receiver.